G2 Level Of Theory Research Articles

Computational study of the thermal decomposition and the thermochemistry of allyl ethers and allyl sulfides

In spite of the several experimental and computational studies on the thermal decomposition of allyl ethers and allyl sulfides, there are still disagreements on aspects of the reaction mechanism, such as the true nature of the transition states and the grade of synchronicity of the reactions. This work presents a computational study of the gas-phase thermolysis reaction of allyl ethers and allyl sulfides substituted at α-carbon, at the M05-2X/6-31+G(d,p) level of theory and a temperature range from 586.15 to 673.15 K. The substituent groups were methyl, ethyl, n-propyl, i-propyl, allyl, benzyl and acetonyl. It was found that the sulfides react faster than the homologous ethers and that the substituent groups with the capacity of delocalize charge increase the reaction rate. Through natural bond orbital calculations, the transition states were characterized. The synchronicities and atomic charges of the studied reactions were determined. A computational study at the G3 level of theory on the thermochemistry of allyl ethers and sulfides was also carried out.

Halodiazirines and halodiazo compounds: a computational study of their thermochemistry and isomerization reaction

A computational study of the isomerization reaction of a series of halodiazirines to halodiazo compounds (cyclic to open-chain RXCN2 species) has been carried out in order to establish the effect of the substituent groups on the isomerization rates and to obtain computational evidence of reaction mechanisms. Fluorine and chlorine were present as the halogen (X) atom, and the groups R=H, CH3, C2H5, n-C3H7, i-C3H7, cyclo-C3H5, phenyl, OCH3 and OH were used. Thermochemical calculations and natural bond orbital analyses were carried out at the B3LYP/6-31+G(d,p) level of theory. The results allowed us to discuss a reaction mechanism that proceeds in two steps: The first is the extrusion of nitrogen and formation of a carbene through a cyclic transition state that promotes the simultaneous breaking of the two C–N bonds, and the second one is described as the rebounding between the carbene and one of the nitrogen atoms of molecular nitrogen, both formed in the first step. The enthalpies of formation of halodiazirines and halodiazoalkanes have been calculated at the G3 level of theory.

Bimolecular Homolytic Substitutions at Nitrogen: An Experimental and Theoretical Study on the Gas-Phase Reactions of Alkyl Radicals with NF3.

The X-ray irradiation of binary mixtures of alkyl iodides R-I (R=CH3 , C2 H5 , or i-C3 H7 radicals) and NF3 produces R-NF2 and R-F. Based on calculations performed at the CCSD(T), MRCI(SD+Q), G3B3, and G3 levels of theory, the former product arises from a bimolecular homolytic substitution reaction (SH 2) by the alkyl radicals R, which attack the N atom of NF3 . This mechanism is consistent with the suppression of R-NF2 by addition of O2 (an efficient alkyl radical scavenger) to the reaction mixture. The R-F product arises from the attack of R to the F atom of NF3 , but additional contributing channels are conceivably involved. The F-atom abstraction is, indeed, considerably more exothermic than the SH 2 reaction, but the involved energy barriers are comparable, and the two processes are comparably fast.

A computational investigation of monosubstituted boroxines(RH2B3O3): structure and formation

Boroxines have emerged as an important class of compounds with diverse applications in many fields. However, there are limited experimental and/or computational structural and thermal data available for these compounds or for their corresponding boronic acids. In this investigation, we report structural parameters for a variety of aliphatic monosubstituted boroxines (RH2B3O3) and their enthalpies of formation via the dehydration reactions from the boronic acids (R–B(OH)2), i.e., R–B(OH)2 + 2H–B(OH)2 → RH2B3O3 + 3H2O. Equilibrium geometries of all the boronic acids and monosubstituted boroxines involved in this article were obtained using second-order Moller–Plesset perturbation theory with the Dunning–Woon aug-cc-pVDZ and aug-cc-pVTZ basis sets; heats of formation were calculated at the G3 level of theory. The specific substituents, R, include all the second- and third-period hydrides, as well as a selection of electron-donating/electron-withdrawing groups.

Are carbon—halogen double and triple bonds possible?

The carbon—halogen and carbon—chalcogen bonding of 84 molecules was investigated utilizing local vibrational modes calculated at the M06‐2X/cc‐pVTZ level of theory including anharmonicity corrections in all cases. The relative bond strength order of each CX or CE bond (X = F, Cl; E = O, S) was derived from the local CX or CE stretching force constant and compared with trends of calculated bond lengths r and bond dissociation energies (BDE) obtained at the G3 level of theory. It is shown that both bond length r and BDE are not reliable bond strength descriptors. The CX double bond is realized for some Cl‐substituted carbenium ions, however, not for the corresponding F‐derivatives. Diatomic CF+ and CCl+ possess fully developed double bonds but not, as suggested in the literature, triple bonds. Halonium ions have fractional (electron‐deficient) CX bonds, which can be stabilized by σ‐donor substituents or by an increased polarizability of the halogen atom as with Cl. Bridged halonium ions are more stable than their acyclic counterparts, which results from more effective two‐electron‐three‐center bonding. © 2014 Wiley Periodicals, Inc.

Computational studies on the thermal decomposition of the CH2FOCHFO radical

Theoretical studies have been carried out on the kinetics and thermochemistry of the thermal decomposition of the CH2FOCHFO radical formed during the photo-oxidation of CH2FOCH2F (HFE-152E) using the dual-level method of obtaining the optimised structure at DFT(M06-2X)/6-311++G(d,p) followed by a single-point energy calculation at the G3 level of theory. The rate constant for different reaction channels involved during the decomposition processes of CH2FOCHFO is evaluated at 298 K and 1 atm using canonical transition-state theory. The results point out that the C–H bond scission is the dominant path involving an energy barrier of 9.5 kcal mol−1 determined at the G3 level of theory. A potential energy diagram is constructed and the results are compared with the data available from the literature for a structurally similar molecule.

Tautomerization in the formation and collision-induced dissociation of alkali metal cation-cytosine complexes

Noncovalent interactions between alkali metal cations and the various low-energy tautomeric forms of cytosine are investigated both experimentally and theoretically. Threshold collision-induced dissociation (CID) of M(+)(cytosine) complexes with Xe is studied using guided ion beam tandem mass spectrometry, where M(+) = Li(+), Na(+), and K(+). In all cases, the only dissociation pathway observed corresponds to endothermic loss of the intact cytosine molecule. The cross-section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) for the M(+)(cytosine) complexes after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and dissociation lifetimes. Ab initio calculations are performed at the MP2(full)/6-31G* level of theory to determine the structures of the neutral cytosine tautomers, the M(+)(cytosine) complexes, and the TSs for unimolecular tautomerization. The molecular parameters derived from these structures are employed for the calculation of the unimolecular rates for tautomerization and the thermochemical analysis of the experimental data. Theoretical BDEs of the various M(+)(cytosine) complexes and the energy barriers for the unimolecular tautomerization of these complexes are determined at MP2(full)/6-311+G(2d,2p) level of theory using the MP2(full)/6-31G* optimized geometries. In addition, BDEs for the Li(+)(cytosine) complexes are also determined at the G3 level of theory. Based upon the tautomeric mixture generated upon thermal vaporization of cytosine, calculated M(+)-cytosine BDEs and barriers to tautomerization for the low-energy tautomeric forms of M(+)(cytosine), and measured thresholds for CID of M(+)(cytosine) complexes, we conclude that tautomerization occurs during both complex formation and CID.

Quantum chemical study of interactions of carbenes and their analogs of the EH2 and EHX types (E = Si, Ge, Sn; X = F, Cl, Br) with HX and H2, respectively: the insertion and substituent exchange reactions

Interactions of carbenes and carbene analogs EH2 and EHX with HX and H2 (E = C, Si, Ge, Sn; X = F, Cl, Br), respectively, were studied by quantum chemical methods. Theoretical analysis of the carbene and silylene systems was carried out at the G3 level of theory using the MP2(full)/6–31G(d) calculated geometries and vibrational frequencies. The stannylene systems were examined at the MP2 level using a modified LANL2DZ basis set for the Sn atoms and the 6–31+G(d,p) basis sets for other atoms. Transformations in the germylene systems were studied within the framework of both approaches, which gave similar results. This allowed one to compare the reaction pathways and their energy profiles for the whole series of systems. In addition to the insertions into the H-X and H-H bonds, the exchange reactions resulting in interconversions of EH2 and EHX can proceed in the systems under consideration. The effects of the nature of the E and X atoms on the reaction barriers and exothermicity of both the insertion and exchange reactions are analyzed. Possible role of radical processes in these systems is assessed.

Chemical synthesis in acetonitrile containing discharges. Insights from photoionization experiments with synchrotron radiation

The photoinduced ion chemistry of acetonitrile is investigated by using tuneable VUV synchrotron radiation in an octupolar ion guide with mass spectrometric detection. The appearance energies and yields of primary photoions and secondary ionic products deriving from self-reactions at thermal collision energies are measured. Comparison of the appearance energies of the different product ions provides insights into the reaction mechanisms. Experimental data are interpreted with the help of theoretical calculations (energies and structures at the G3 level of theory) for the various isomers of the most relevant ionic products. We find that the ion C2H2N+ is formed at the threshold with a cyclic structure, but only its linear isomer (lying 0.47eV higher in energy) can react with acetonitrile to form CH2CNCH2+ plus HCN via a cyclic transition state.

Kinetics of Al + H2O Reaction: Theoretical Study

Quantum chemical calculations were carried out to study the reaction of Al atom in the ground electronic state with H(2)O molecule. Examination of the potential energy surface revealed that the Al + H(2)O → AlO + H(2) reaction must be treated as a complex process involving two steps: Al + H(2)O → AlOH + H and AlOH + H → AlO + H(2). Activation barriers for these elementary reaction channels were calculated at B3LYP/6-311+G(3df,2p), CBS-QB3, and G3 levels of theory, and appropriate rate constants were estimated by using a canonical variational theory. Theoretical analysis exhibited that the rate constant for the Al + H(2)O → products reaction measured by McClean et al. must be associated with the Al + H(2)O → AlOH + H reaction path only. The process of direct HAlOH formation was found to be negligible at a pressure smaller than 100 atm.

Theoretical Study on Decomposition of CF3OH Catalyzed by Water Dimer and Ammonia

The G3 and CBS-QB3 theoretical methods are employed to study the decomposition of CF3OH into FCFO and HF by water, water dimmer, and ammonia. The decomposition of CF3OH into FCFO and HF is unlikely to occur in the atmosphere due to the high activated energy of 88.7 kJ/mol at the G3 level of theory. However, the computed results predict that the barrier for unimolecular decomposition of CF3OH is decreased to 25.1 kJ/mol from 188.7 kJ/mol with the aid of NH3 at the G3 level of theory, which shows that the ammonia play a strong catalytic effect on the split of CF3OH. In addition, the calculated rate constants show that the decomposition of CF3OH by NH3 is faster than those of H2O and the water dimmer by 109 and 105 times respectively. The rate constants combined with the corresponding concentrations of these species demonstrate that the reaction CF3OH with NH3 via TS4 is of great importance for the decomposition of CF3OH in the atmosphere.

Theoretical Investigation of Carbon–Sulfur Triple Bonds

By means of first principle calculations we have investigated a set of molecules that are presumed to contain carbon-sulfur triple bonds, namely HCSOH, H(3)SCH, cis-FCSF, F(3)CCSF(3), and F(5)SCSF(3). For HCSOH, FCSF, and H(3)SCH we used the CCSD(T) methodology and the correlation-consistent basis sets. On the other hand, F(3)CCSF(3) and F(5)SCSF(3) were studied at the B3LYP, M06-2X, MP2, and G3 levels of theory. We found that none of these molecules display a carbon-sulfur adiabatic bond dissociation energy (ABDE) as strong as diatomic CS (170.5 kcal mol(-1)), or a diabatic bond dissociation energy (DBDE) larger than the one found in SCO (212.0 kcal mol(-1)), although the DBDE of FCSF comes quite close at 208.3 kcal mol(-1). The CS ABDEs of F(3)CCSF(3), F(5)SCSF(3), and H(3)SCH are comparable to that of a single C-S bond. In contrast with the experimental results, F(3)CCSF(3) and F(5)SCSF(3) are predicted to be linear with C(3v) and C(s) symmetry, respectively, at the B3LYP/6-311+G(3df,2p) level. MP2/6-311+G(2df,2p) calculations support the C(3v) symmetry for F(3)CCSF(3), despite F(5)SCSF(3) not having a perfect linear structure; the CSC angle is 174.6°, which is nearly 20° larger than the experimental value. The analysis of the carbene structures of HCSOH and H(3)SCH revealed that they are not significant, because the triplet state is dissociative in these cases. However, for F(3)CCSF(3) and F(5)SCSF(3) , the carbene triplet states lie 0.81 and 0.77 eV above the singlet state, respectively. In the same vein, our investigation supports the presence of a strong double bond for HCSOH. The conflicting evidence available for F(3)CCSF(3) and F(5)SCSF(3) makes it very difficult to determine the nature of the CS bonds. However, the bond dissociation energies and the singlet-triplet splittings clearly suggest that these compounds should be considered as masked sulfinylcarbenes. The analysis of the bond dissociation energies challenges the existence of a triple bond in these five molecules, but from a strictly thermodynamic standpoint, cis-FCSF is found to be the candidate most likely to exhibit triple-bond character.

Structure and Thermochemical Properties of 2-Methoxyfuran, 3-Methoxyfuran, and Their Carbon-Centered Radicals Using Computational Chemistry

Methoxyfurans are known components in a number of biofuel synthesis processes and their thermochemical properties are important to the stability, reaction paths, and chemical kinetics of these species. Enthalpies (DeltaH degrees (f298)), entropies (S degrees (298)), and heat capacities (C(p)(T)) are reported for 2-methoxyfuran and 3-methoxyfuran, cyclic ethers with possible biofuel implications, and their radicals corresponding to loss of hydrogen atoms. Standard enthalpies of formation are calculated at the B3LYP/6-31G(d,p), B3LYP/6-311G(2d,2p), CBS-QB3, G3MP2B3, and G3 levels of theory with isodesmic reactions to minimize calculation errors. Structures, vibrational frequencies, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) density functional level and are used to determine the entropy and heat capacities. The recommended ideal gas phase enthalpy of formation, from the average of the CBS-QB3 and G3MP2B3 levels of theory, for 2-methoxyfuran is -45.0 kcal mol(-1) and for 3-methoxyfuran is -41.1 kcal mol(-1). Bond dissociation energies are also calculated. The C-H bonds of the furan ring are approximately 120 kcal mol(-1), which is consistent with recent data on several alkylfurans; they are significantly stronger than non-aromatic, stable heterocyclic structures. The bond energy decreases to 98 kcal mol(-1) for the methoxy-methyl C-H bonds making this methyl site a favorable abstraction target and an important site for initial decomposition paths during combustion. Group additivity for furan is discussed and groups for furan and methoxyfuran carbon radicals are derived.

THE IONIZATION ENERGIES AND SIMULATED PHOTOELECTRON SPECTRA OF HPCN AND HNCP

The structures, vibrational frequencies and adiabatic ionization energies of HPCN and HNCP are calculated at several levels of theory. The adiabatic ionization energy of HPCN is found to be 10.16 eV at the G3 level of theory. The singlet state of HPCN+[Formula: see text] is found to be approximately 1.3 eV below the lowest energy triplet state (ã3A ″). Both states have a bent equilibrium molecular geometry. The adiabatic ionization energy for HNCP is calculated to be 8.30 eV at the G3 level of theory. In contrast to HPCN+, the triplet state [Formula: see text] of HNCP+is lower in energy than that of the singlet state (ã1A ′) by approximately 1 eV. Also, the triplet state of HNCP+is linear in contrast to that of HPCN+due to the larger interaction between neighboring 2p orbitals on the central atoms in HNCP+relative to the interaction between the 3p and 2p orbitals on the central atoms in HPCN+. Simulated photoelectron spectra (PES) are presented for the transitions producing both the singlet and triplet ion states of both isomers. As predicted by the dramatic geometry change in the case of [Formula: see text], there is a long progression in the bending mode of the cation in the simulated PES.

Does Tetrahydrofuran Ring Open upon Ionization and Dissociation? A TPES and TPEPICO Investigation

The threshold photoelectron spectrum (TPES) of tetrahydrofuran (THF) is compared to that of the unsaturated furan molecule. In general, there is a similarity in the orbital ionization profile for the two species, though unlike furan, THF exhibits (modest) vibrational detail only in the (9b)(-1) X (2)B band. An adiabatic ionization energy of 9.445 +/- 0.010 eV has been derived from the onset of the TPES spectrum. Threshold photoelectron photoion coincidence spectroscopy was used to explore the loss of a hydrogen atom from ionized THF over the photon energy range of 9.9-10.4 eV. RRKM fitting of the resulting breakdown curves yields an E(0) of 0.85 +/- 0.03 eV (82 +/- 3 kJ mol(-1)) (AE = 10.30 +/- 0.04 eV). If the G3 IE of 9.48 eV is used to convert the experimental data from photon energy to THF ion internal energy, E(0) = 0.81 +/- 0.01 eV (78 +/- 1 kJ mol(-1)). The latter value is closer to the G3 E(0) of 72 kJ mol(-1) for the formation of the cyclic ion 1. A variety of ring-opening reactions were also probed at the B3-LYP/6-31+G(d) and G3 levels of theory. The distonic isomer (*)CH(2)CH(2)CH(2)OCH(2)(+) lies 70 kJ mol(-1) higher than ionized THF, which places it within 1 kJ mol(-1) of the threshold for the dissociation to 1. All of the probed H-loss products from the distonic isomer (which includes singlet and triplet species) lie significantly higher in energy than ion 1, eliminating the possibility that ionized THF dissociates to m/z 71 via a ring-opening reaction in the present experiment. The derived Delta(double dagger)S value for the dissociation, 8 +/- 5 J K(-1) mol(-1), is also consistent with the formation of 1. The experimentally derived E(0) values can be used to derive the Delta(f)H(o)(0) for ion 1. Together with the Delta(f)H(o)(0) values for the THF ion (752.0 +/- 2 kJ mol(-1), derived from the neutral Delta(f)H(o)(0) of -154.9 +/- 0.7 kJ mol(-1) and experimental IE of 9.445 +/- 0.010 eV) and H atom (218.5 kJ mol(-1)) our E(0) of 82 +/- 3 kJ mol(-1) yields a Delta(f)H(o)(0) for ion 1 of 620 +/- 4 kJ mol(-1) (Delta(f)H(o)(298) = 594 +/- 4 kJ mol(-1)), in good agreement with the G3 Delta(f)H(o)(0) of 621 kJ mol(-1). Appearance energies for all fragment ions up to photon energies of 34 eV are also reported and discussed in comparison with the available literature.

Isomerization Barriers and Strain Energies of Selected Dihydropyridines and Pyrans with Trans Double Bonds

The stability of cis,trans-dihydropyridines and cis,trans-pyrans has been studied using ab initio methods. The strain introduced by the trans double bond has been determined relative to the cis,cis-isomers and introduces 58-69 kcal x mol(-1) of strain energy, at the G3 level of theory, depending on the particular isomer. Double bond rotation barriers have been calculated at the MRMP2/MCSCF level and range from 2.2 kcal x mol(-1) to 11.0 kcal x mol(-1), significantly lower than butadiene (50.3 kcal x mol(-1)). Evidence of resonance through delocalization of the pi electrons is present for the conjugated double bond isomers which lowers the activation barriers. The transition states for trans double bond rotation have significant biradical character but markedly less than that for butadiene. The early transition states with H-C=C-H dihedral angles of 130-150 degrees, as opposed to 90 degrees for butadiene, are consistent with the reduction in the natural orbital occupation numbers. We could not locate a minimum for a structure having both double bonds in the trans configuration and so report that one trans bond is the most the rings can accommodate.

Experimental and theoretical studies of electrochemical characteristics of 3,4-dihydroxyphenylacetic acid (DOPAC)

The electrochemical behavior of 3,4-dihydroxyphenylacetic acid (DOPAC) in aqueous solutions with different pHs and DOPAC concentrations was studied by cyclic voltammetry at various potential scan rates. The electro-oxidation of DOPAC involves a transfer of two electrons and two protons in solutions of pH < 8.0 and a transfer of two electrons and one proton in solutions of pH > 8.0, in agreement with the one-step two-electron mechanism. The cyclic voltammetry indicated that the process of electro-oxidation of DOPAC follows an E rC i mechanism. The standard redox potential, E 0′, two-proton–two-electron and one-proton–two-electron processes of DOPAC were determined as 0.585 and 0.357 V versus saturated calomel electrode (SCE), respectively. The acidic dissociation constant of DOPAC was also obtained. Also, the diffusion coefficient of DOPAC was calculated as 9.2 × 10 −6 cm 2 s −1 for the experimental conditions, using chronoamperometric results. The standard reduction potentials and the second acidic constant of DOPAC have been also calculated using standard ab initio calculations at the G3 level of theory in conjunction with a continuum solvation model. The theoretical values are in good agreement with experiment.

Nitro–Nitrite Isomerization and Transition State Switching in the Dissociation of Ionized Nitromethane: A Threshold Photoelectron–Photoion Coincidence Spectroscopy Study

Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been employed to investigate the competition between bond cleavage and rearrangement reactions in the dissociation of ionized nitromethane, 1. Modeling TPEPICO breakdown diagrams with a combination of RRKM theory and ab initio calculations at the G3 level of theory allowed the derivation of the activation energy for the isomerisation of 1 to ionized methyl nitrite, 2, 82 kJ mol(-1). In addition, evidence was found for a transition state switch in the bond cleavage reaction in 1 leading to CH(3)(*) + NO(2)(+). As internal energy increases, the effective transition state for this reaction becomes tighter (i.e. is characterized by a lower entropy of activation, Delta(double dagger)S). Fitted thresholds for NO(+) and CH(2)OHO(+) ions, originating from the isomeric methyl nitrite ion, are consistent with G3 level ab initio calculations.

Ring Strain Energy in the Cyclooctyl System. The Effect of Strain Energy on [3 + 2] Cycloaddition Reactions with Azides

Ring strain energies (SEs) and enthalpies of hydrogenation (DeltaH(hyd)) of a series of E- and Z-alkenes, cyclic alkynes and allenes (C(5)-C(9)) are computed at the G3 level of theory. The SE for cycloheptyne, cyclohexyne, and cyclopentyne are calculated to be 25.4, 40.1, and 48.4 kcal/mol, respectively. The SE for E-cycloheptene and E-cyclohexene are calculated to be 25.2 and 49.3 kcal/mol (G3). The SE of cyclooctyne is 2.0 kcal/mol greater than that of E-cyclooctene (17.9 kcal/mol) but only 7.7 kcal/mol greater than that of cyclooctane. The SE of 3,3-difluorocyclooctyne (DIFO) is predicted to be slightly reduced (DeltaSE = 2.6 kcal/mol) relative to the parent cyclooctyne to 17.3 kcal/mol. The SE and DeltaH(hyd) are correlated with activation barriers for the [3 + 2] cycloaddition of a series of azides to E- and Z-cycloalkenes and alkynes at the G3 level of theory. The energy barrier for the cycloaddition of methyl azide to cyclooctyne is 9.2 kcal/mol lower than addition to 4-octyne and 3.1 kcal/mol lower for reaction with E-cyclooctene. The activation energies for [3 + 2] cycloaddition of benzyl azide and acetamido azide ((2)HN(C=O)CH(2)-N(3)) to DIFO are 2.3 and 5.3 kcal/mol lower in energy than cycloaddition to cyclooctyne [B3LYP/6-311+G(3df,2p)].

Halogen switching of azacarbenes C2NH ground states at ab initio and DFT levels

AbstractRelative stabilities and singlet–triplet energy differences are calculated for 24 C2NX azacarbenes (where X is H, F, Cl, and Br). Three skeletal arrangements are employed including azacyclopropenylidene, [(imino)methylene]carbene, and cyanocarbene. Halogens appear to alternate the electronic ground states of C2NH azacarbenes, from triplet to singlet states, at MP3/6‐311++G**, B1LYP/6‐311++G**, B3LYP/6‐311++G**, MP2/6‐311++G**, MP4(SDTQ)/6‐311++G**, QCISD(T)/6‐311++G**, CCSD(T)/6‐311++G**, CCSD(T)/cc‐pVTZ, G1, and G2 levels of theory. The aromatic characters of singlet cyclic azacyclopropenylidenes are measured using GIAO–NICS calculations. Linear correlations are found between the B3LYP/6‐311++G** calculated LUMO–HOMO energy gaps (ΔEHOMO ‐ LUMO) of the singlet carbenes versus their corresponding singlet–triplet energy separations (ΔE). Electrophilic characters are found for all singlet azacarbenes in their addition reactions to alkenes with the highest electrophilicity being exhibited for X = F. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:377–388, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20442